OTHER FORMS OF LIME
_Air-Slaked Lime._ A pure limestone is a carbonate, and the chemical
formula is CaCO_3. When it is burned, the carbon dioxide (CO_2) is
driven off, leaving CaO, which is calcium oxide, called fresh burned
lime. In this process 44 pounds of a stone weighing 100 pounds passes
into the air, and there remain 56 pounds of lime. When it air-slakes, it
takes back the carbon dioxide from the air, and the new product becomes
CaCO3, or carbonate of lime, and regains its original weight of 100
pounds. This is what would happen if the process were complete, and it
is nearly so when the exposure to the air is as perfect as possible.
Fifty-six pounds of valuable material are in the 100 pounds of
air-slaked lime, just as is the case with limestone, and there is no
difference in effectiveness except in so far as the air-slaked material
is absolutely fine and available, while most pulverized limestone is
less so. In making purchase for use of land the buyer cannot afford to
make any appreciable difference in price in favor of air-slaked lime,
as compared with a fine stone.
_Air-Slaking a Slow Process._ Lime changes to an air-slaked condition
slowly unless it has full exposure. Old heaps will remain in hydrate
form for many years, excepting the outside coat, which excludes the air.
Complete air-slaking would not reduce ability to correct soil acidity,
the total amount of calcium and magnesium remaining constant, but weight
would be added in the slaking, and therefore the value per ton would be
reduced. The slowness with which air-slaking proceeds gives reason to
expect that any bulk of old lime may contain a considerable percentage
of the hydrate, and therefore have greater strength than a true
carbonate like limestone. This is a consideration of value to a buyer.
_Agricultural Lime._ Some manufacturers have found in the demand for
lime by farmers an opportunity of disposing of much material that would
not be satisfactory to manufacturers and builders. In some cases this
so-called agricultural lime is sold at a price that is not beyond value,
but it varies much in its content of pure lime. If the unburned cores
of kilns are ground up, the material simply retains the value of
unburned stone. Any air-slaked material put into it has like value.
Forkings, ground up, have less value, and sometimes no value at all.
Some better material may go into this mixture that is given the name
"agricultural lime," and the product cannot be standardized or have a
valuation given it that would be true for another lot.
Some manufacturers are marketing limes of fair values under this
designation, but the values change as the material changes. There are
other manufacturers who are putting poor stuff on the market. Unless one
knows the manufacturer and his processes, he should not pay a great deal
for "agricultural lime." It is much better to buy a high-grade lime or
limestone that is more nearly constant in composition. When the word
"agricultural" is part of the brand, there is assurance that the
percentage of waste stuff in it is relatively high. Unless one knows to
the contrary, he should assume that a ton of finely pulverized limestone
is worth more per ton than "agricultural lime."
_Marl._ Marls vary in composition, as limestones do, but there are beds
of chalky marl that contain very little clay and sand and are nearly a
pure carbonate. It is only marls of high degree of purity that can be
put on the market with profit, but beds of less pure marl furnish
dressings for farms of the locality in many sections of the country.
Some of these inferior marls have had so much clay and sand mixed with
the lime carbonate that dressings must be heavy. The best lime marls
provide excellent material for the correction of soil acidity, the
actual value per ton being practically the same as that of the finest
pulverized limestone. Some dealers in marl make extravagant claims for
their goods, but any farmer may easily put these claims to the test and
learn that he should not expect more than a fairly good carbonate of
lime can do.
Marl improves the physical condition of stiff soils only when used in
large amount per acre, and this is true of any carbonate form, such as
limestone. Little effect upon physical condition should be expected from
the light application usually given when marl is purchased and
transported some distance to the farm. The chalk marl on the market is
used to correct soil acidity, and at the best it is worth only what good
lime carbonate is worth. It has no hidden virtues, and cannot take the
place of fertilizers. It is an excellent means of meeting the
lime-requirement of land when bought right, and its fine division makes
it distinctly superior to coarse stone.
There should be no confusion of a lime marl with the so-called "green
sand" marl. The latter is low in lime, and may be acid, the value of the
marl being in a considerable percentage of plant food contained.
_Oyster Shell._ Ground oyster shell is a good source of carbonate of
lime. The percentage falls below that of limestone, but in addition
there is a little nitrogen and phosphoric acid. An analysis of a good
quality of oyster shell, as found on the market, will show 90% carbonate
of lime.
Burned oyster shell has something near the same composition as lime made
from stone, but it goes back to hydrate and air-slaked forms rapidly.
There is no large amount of burned shell lime on the market, the
material known as shell lime being the ground shell, or lime carbonate.
_Wood Ashes._ A large supply of lime in excellent form was afforded by
hardwood ashes, but this product has ceased to have any important value
to our agriculture. The chief supply on the market is low in quality,
containing moisture and dirt in considerable amount, the form of lime
being changed from an oxide to the hydrate and carbonate.
_Gas Lime._ Prof. E. B. Voorhees, in "First Principles of Agriculture,"
says: "Gas lime is also frequently used as manure; in gas works,
quicklime is used for removing the impurities from the gas. Gas lime,
therefore, varies considerably in composition, and consists really of a
mixture of slaked lime, or calcium hydrate, and carbonate of lime,
together with sulfites and sulfides of lime. These last are injurious to
young plant life, and gas lime should be applied long before the crop is
planted, or at least exposed to the air some time before its
application. The action of air converts the poisonous substances in it
into non-injurious products. Gas lime contains on an average 40% of
calcium oxide, and usually a small percentage of nitrogen."
_Lime After Magnesium Removal._ A by-product in the removal of
magnesium from a magnesian limestone is an excellent material for
correction of soil acidity, on account of its physical condition. Its
exposure to the air causes much of the hydrate to change to an
air-slaked form, and its value per ton lies somewhere between that of
very finely pulverized limestone and hydrated lime.